Digital mobile stations are generally provided with a mobile station specific equipment identity code saved permanently in the mobile station and with an operator/customer specific user code saved in a memory card or a smart card. In mobile communications systems the rights of use of mobile phone users connected to a telecommunication network and the utilization rate of the network must be controlled because of the invoicing relating to the use of the telecommunication network. This control is implemented with the help of codes defined for the users and the operators of the telecommunication network. In addition to the rights of the users, also the right of a mobile station to be used in the network has to be controlled so as to prevent the use of stolen mobile stations and those found to be defective. This has been arranged with a mobile equipment identity code.
The use of a digital mobile station can be restricted to a telecommunication network of a certain operator only. Thus, it is possible to make agreements between the user of the mobile station and the operator, according to which the mobile station is owned by the operator or the purchase cost of the mobile station is compensated to the customer by the operator, in which cases it is in the operator's interest to restrict the use of the mobile station to the operator's own network only. This function is also based on the use of the above mentioned codes.
In the following description the control of the right of use of a mobile station will be exemplified in the GSM system (Global System for Mobile Communications). The GSM system has been described in more detail in the following publication: 1! M. Mouly, M-B. Pautet: The GSM System for Mobile Communications, 1992. In the GSM system the International Mobile Equipment Identity code IMEI is saved in the permanent memory of the mobile station when manufacturing the device. The code includes a Type Approval Code TAC, a Final Assembly Code FAC and a serial number. The code is for each mobile station individually unique. When starting a communication or during it, the data transmission network can request the IMEI code of the mobile station and compare it with a register of authorized and/or unauthorized mobile stations. The IMEI code is used, for example, for preventing the use of stolen or defective mobile stations.
In the GSM system the codes relative to the operator and the user of the telecommunication network have been stored in the so called SIM module (Subscriber Identity Module). The SIM module is generally a part of a so called SIM card having an electric coupling for attaching the card to the mobile station. The SIM module includes, by example, the information required for the communication concerning the user, the telecommunication network to be used, and the encryption of the radio data transmission. The SIM module includes also the customer's invoicing data to be transferred to the telecommunication network simultaneously with the call from the mobile station. In this way the user can use the SIM card as needed in different mobile stations, unless the use of the mobile station is restricted, for example, so that the mobile station can be used only if a SIM card of a certain operator is inserted into it. If the use of the mobile station or some of its features is restricted in the above mentioned way, the restriction will be implemented with codes stored in the SIM module.
In the following the "lock closing" of the mobile station means a function where the use of some or all of the features of the mobile station requires that the certain data saved in the mobile station and in the SIM module are identical. Correspondingly, the "lock opening" means that the lock in question is removed. A mobile station is usually locked when it is sold, so that the user makes an agreement with the operator of a certain telecommunication network. In that case a lock closing parameter, which is a predefined field to be read from the SIM module, is saved in the memory of the mobile station. Codes generally used for the lock closing are the so called International Mobile Subscriber Identity code IMSI, and two so called Group Identification Level codes GID1 and GID2. The IMSI code saved in the SIM module consists of an operator specific part and a card specific part. The length of the IMSI code is 15 digits, in which the operator identification part consists of 5 digits. The mentioned two group identification levels GID1 and GID2 are meant as special codes of the operator, service provider, or customer, and they can be programmed into the mobile station only by the operator. Both group identification levels consist of e.g. two bytes. Neither the IMSI code of the SIM module nor the contents of the group identification levels can be changed after being saved.
The user of the mobile station can open the lock of the mobile station by inputting to the mobile station the lock open password given by the operator. The mobile station then compares the entered lock open password with the comparative password saved in the permanent memory of the mobile station, and if the passwords are identical the processor of the mobile station opens the lock. The operator calculates the above-mentioned mobile station specific lock open password with a calculation program.
There are several problems relating to the known lock closing methods concerning the lock opening. If one and the same lock open password calculation method is used, regardless of the operator, it is possible, by using the same lock open password calculation program, to find out the lock open passwords of mobile stations connected to networks of all operators.
Another known solution is to use an operator specific lock open password calculation program, whereby the above-mentioned problem is avoided. In this case it is not possible, however, to save comparative passwords to the mobile stations when manufacturing, and they have to be instead programmed when connecting the mobile station to a certain operator's network. This causes extra work for the sales organization, as well as the need to invest in numerous programming devices. Further, the contents of the memory can be read after programming, which increases the risk of unauthorized lock opening.
A third possibility is to program every operator's data to each mobile station during manufacture. The problem with this solution is, however, that there are a large number of operators and thus the permanent memory required for saving the data in the mobile station would have to be unreasonably large. Further, the memory requirement will increase if every operator uses several different locking levels by which the mobile station can be locked to alternative codes of the SIM module. Also, new operators can come on the market, in which case only mobile stations that are manufactured after the codes for the respective operators have been agreed, could be connected to the networks of the new operators. That would create problems in marketing, as there would be many versions of devices.